In most vehicles, fuel level is measured by means of a float attached to the end of a pivot arm. The arm is connected to the wiper of a variable resistor or potentiometer which provides an electrical signal to operate a display. Typically, one end of the variable resistor is connected to ground. The position of the float is determined by the fuel level in the tank. The wiper is either directly connected to an air core gauge, a bimetal gauge, or to an electronic circuit which indirectly feeds the display means. In newer vehicles, baffles in tanks used to subdue fuel slosh have been substantially removed to lower costs and to allow for inexpensive plastic tanks to replace metal ones. Without baffles, fuel sloshes more in response to vehicle maneuvers which makes accurate estimation of fuel level difficult and calls for electronic circuits to interpret sensor data.
The dimensions of tanks vary from vehicle to vehicle. Plastic tanks are particularly dimensionally unstable and the height of a tank in a given vehicle may change substantially over time and change with temperature as well. Many types of older fuel level sensor assemblies were mounted to the top of the tank causing the float position to also be mechanically referenced to the top of the tank. Variations in tank dimensions both within a given vehicle and from vehicle to vehicle leads to uncertainty of the float position relative to the bottom of the tank. In this situation, the amount of fuel remaining at low levels is not easy to determine accurately. The low fuel condition, however, is important to the driver for obvious reasons. In many cases to correct for low fuel inaccuracies, fuel float assemblies have been redesigned to provide a bottom referenced system. While fuel float assemblies are still mounted to the top of the tank, the float/pivot/sensor portions of the assemblies have been spring-biased against the bottom of the tanks to insure they remain in firm contact with tank bottoms even as the height of the tank varies throughout the life of the vehicle. Of course, in this situation, the amount of fuel in a full tank cannot be accurately determined. Once filled with gas, a vehicle with a bottom referenced sender could in some circumstances be driven for considerable distances before the fuel gauge registered any lowering in the fuel level. It is preferred, however, that accurate fuel levels be reported for all conditions, not only for low fuel. Although the physical reference of the sender is the tank bottom, variances in sender manufacture can result in different values for an empty tank. A knowledge of the actual sender value for an empty condition would increase the accuracy of the indicated fuel amount.